New Zealand has had a 600V HVDC link from the South Island to the North Island for quite some years - most of the power is generated by the hydro lakes in the south, then delivered via a 610km link to a substation in the north, where it is fed into the normal high-voltage transmission grid.

http://en.wikipedia.org/wiki/HVDC_Inter-Island

So it is a proven technology to have this as a long distance link (been in operation since 1965) and it is capable of bi-directional power transfer as well.

I'm curious where you get the "600V" number from -- is that 600 volts? According to the Wikipedia page you linked the system uses "+270kV and −350kV", which is in the range I would expect. Maybe you meant 620kV, the difference between +270kV and -350kV?

My apologies, that must've been some confusion in my mind... it was a 600MW HVDC link... although it is higher now, and works are underway to bring it up to somewhere near 1.4GW.

HVDC definitely makes sense for connecting islands together. The cable goes underwater, and in water, the capacitance of the cable is much increased by the greater dielectric constant of the water, which makes AC transmission less efficient.

You know I've never heard of HVDC until I read this. So was Edison right all along?

Not really. The AC transformer was the most efficient way to adapt voltages until very recently. The IGBT and GTO thyristors are the technology that makes something like HVDC affordable today, and they are very modern developments.

Mercury-arc rectifiers (the technology replaced with IGBTs and GTOs) were very large and expensive, and also less efficient (even more so before the 1930s or so) so HVDC only made sense for submarine cables (which have huge capacitive losses). With IGBTs and GTO thyristors it start to become feasible to e.g. do an HVDC line across a continent.

As an example the HVDC inter-island was built using mercury-arc valves (it included a submarine leg), but they have since benn replaced with solid-state devices.

HVDC is also a good to tie two separate grids together; since they will be on different time-bases you can't just directly AC couple them.

No. AC is still far more useful in many key scenarios [1]. However, for long-distance power transmission, lots of money could be saved (and jobs created in doing so) by switching from AC to HVDC.

[1] http://en.wikipedia.org/wiki/High-voltage_direct_current#Dis... (note: some of these are due to AC parts having economies of scale and decades of efficiencies and process that HVDC may not be able to tap into without widescale implementation)